This invention relates to an electron gun for use in cathode ray tubes and especially an electron gun of unipotential type.
The electron gun of this invention is applicable to known monochromatic and/or color cathode ray tubes of television receiving sets. As is well known in the art, the envelope of the cathode ray tube of this type is constituted by a panel, a funnel and a neck. On the inner surface of the panel is formed a phosphor screen upon which an electron beam impinges to cause the phosphor screen to luminesce. Inside the neck is provided an electron gun including a plurality of electrodes which emits an electron beam of the desired intensity under the control of video signals. Typically, a color picture tube contains in its neck three electron guns for emitting three electron beams. Within the panel is supported a color selection electrode or a shadow mask having a plurality of apertures for passing the electron beams. These apertures have a predetermined configuration and arranged in the form of a predetermined pattern. On the inner surface of the panel of a color picture tube is formed a phosphor screen containing phosphors for causing red, green and blue color luminescences, these phosphors being arranged in a pattern which is determined by the arrangement of electron guns and the color selection electrode apertures of the predetermined configuration.
The electron gun comprises an electron beam emitting member which emits a controlled amount of an electron beam under the control of an external control signal, for example a video signal, and a group of electrodes which constitutes a main electron lens for focusing the electron beam on the phosphor screen.
Especially, in a unipotential type electron gun, the electrode group constituting the main lens comprises three cylindrical electrodes which are arranged coaxially with predetermined gaps therebetween. The same anode voltage is impressed upon the electrodes on both sides of the assembly whereas a suitable focusing voltage is impressed upon the intermediate electrode, these electrodes greatly governing the focusing characteristic of the electron gun.
However, in the prior art unipotential type electron gun, the length of the intermediate electrode is shorter than that of the electrodes on both sides and the focusing voltage applied to the intermediate electrode is usually set to be substantially zero. With such electron gun, there are such defects that the spherical abberration is large and the beam spot diameter is not sufficiently small in a practical range of current.
Above description refers to the defects of the unipotential type electron gun for use in a monochromatic cathode ray tube, but the unipotential type electron gun assembly for use in a color picture tube wherein three electron guns each having a similar construction to that of the monochromatic cathode ray tube has also the same defects.